1. Field of the Invention
The present invention relates to an inkjet printing apparatus and a printing method, and more particularly relates to an inkjet printing apparatus and a printing method that are suitably applied to inkjet printing in which a printing head moves to the same image area on a printing medium one or more times to achieve the printing. Moreover, the present invention can be applied to any apparatus that performs printing on a printing medium such as paper, cloth, leather, nonwoven fabric, OHP plastic film, even metal or the like. Specifically, the applicable apparatuses include office machines such as a printer, a copy machine and a facsimile machine, and industrial manufacturing machines such as a print machine and a printing machine.
2. Description of the Related Art
There is an inkjet printing method that employs a so-called serial printing method that performs printing an image on a printing medium by alternately repeating operation of ejecting ink while moving a printing head and operation of conveying the printing medium. In such an inkjet printing method, a band-like image area (band) is formed by one movement (pass) of the printing head.
When the printing is performed in one pass on a region printable by one movement of the printing head on a printing medium, an amount of ink applied to the printing medium at a time is larger than that applied when multi-pass printing for forming an image in a plurality of passes is performed. Hence, in the one pass printing, a stripe is more likely to occur although the degree thereof depends on the printing medium and the property of the ink, because a printing density becomes high at adjacent ends (a boundary between adjacent bands). This stripe is also called a “connecting stripe” or “boundary stripe”. When the “boundary stripes” are produced, the printing quality is likely to be reduced to an extremely low level.
To solve the foregoing problem, in Japanese Patent Laid-Open No. 2002-96460, there is proposed a method which reduces boundary stripes by reducing the amount of ink applied to the vicinity of the boundary. In the technology disclosed in Japanese Patent Laid-Open No. 2002-96460, boundary stripes are reduced by determining a hue in the region of interest which is the target of the above-described reduction processing and by reducing the amount of ink at a reduction rate corresponding to the hue (for convenience, hereinafter referred to as boundary processing based on hue determination).
However, as the higher quality of color images has been required in recent years in particular, the number of colors (the number of types of ink) that are used in printing has tended to increase; also when performing processing to reduce the boundary stripes, it tends to be insufficient to target only basic colors.
For example, Japanese Patent Laid-Open No. 2002-96460 discloses a configuration in which not only color inks of chromatic colors, which are three primary colors (cyan, magenta and yellow) of printing, but also an ink of black as an achromatic color is used. Here, Japanese Patent Laid-Open No. 2002-96460 only discloses the processing to reduce the amount of color inks to be applied without consideration of the black ink. This is because, when data in which black and colors are mixed is printed, black is printed at least one scan earlier than colors are printed. Specifically, when colors are printed, black ink has already been fixed on the printing medium, and this reduces the effects of the black ink on the production of boundary stripes. However, in order to effectively reduce the boundary stripes, it is necessary to take black into consideration. In this case, it can be considered that the above-described boundary processing based on hue determination is also performed on the data of the black ink. However, since the black ink substantially has no hue, in order to effectively reduce the boundary stripes attributable to the black ink, instead of performing the boundary processing based on hue determination, it is preferable to design boundary processing unique to the black ink having no hue. In the black ink actually used, its colorant (a dye or a pigment) itself has a slight color, and thus it does not necessarily have a hue at all (in other words, a=b=0 in a Lab color space). However, since the color of the black ink is a color that is present in the vicinity of an L axis in the Lab color space, it may be treated as substantially having no hue.